This application is a continuation of PCT/EP00/08484 filed Aug. 31, 2000 which published as WO 01/17675 on Mar. 15, 2001 and claims benefit to German application 199 43 167.1 filed Sep. 9, 1999.
The invention relates to a method for producing supported chromium catalysts for the polymerisation of olefins, and to the catalysts obtainable by the method according to the invention.
For the purposes of the invention, the term supported chromium catalysts denotes chromium catalysts as used for the polymerisation of xcex1-olefins (cf. M. P. McDaniel, Adv. Cat. 33, pages 47 to 98 (1985)).
Polymerisation catalysts which comprise silica gel or modified silica gel as support material and chromium as active component play a major role in the production of high-density polyethylene (HD-PE).
The conditions during production of the supports and catalysts determine the chemical composition, pore structure, particle size and shape of the catalysts. Before the polymerisation, the catalysts are activated at high temperatures under oxidizing conditions in order to stabilise chromium on the catalyst surface. The catalyst is subsequently reduced by addition of ethene or additional reducing agents in order to form the catalytically active species which catalyses the polymerisation.
The production of supported chromium catalysts of this type is usually carried out in two steps. In a first step, the support material is firstly brought into contact with a soluble chromium compound in a suitable solvent. In a second step, the mixture of support and chromium compound is subsequently calcined in a stream of air or in a stream of oxygen at high temperatures, approximately between 300 and 1100xc2x0 C. The nature of the support material is of major importance.
Suitable supports can be produced as described in DE-A 25 40 279. This process relates to the preparation of a silicic acid hydrogel having a solids content of from 10 to 25% by weight (calculated as SiO2) whose particles are substantially spherical,
by introducing a sodium or calcium water-glass solution into a swirling stream of a mineral acid, both longitudinally and tangentially to the stream,
spraying the resultant silicic acid hydrosol in drop form into a gaseous medium,
allowing the hydrosol to solidify in a gaseous medium to give hydrogel particles,
washing the resultant, substantially spherical hydrogel particles in order to free the hydrogel from salts,
extracting the hydrogel particles with an alcohol until at least 60% of the water present in the hydrogel has been removed,
drying the resultant hydrogel at temperatures of xe2x89xa6160xc2x0 C. at atmospheric pressure using an inert entraining gas until the residual alcohol content is below 10% (xerogel formation),
setting the desired particle size of the resultant xerogel.
On the other hand, the silicon aluminium phosphate supports disclosed in DE-A 42 28 883 can be used.
The chromium oxide catalysts are generally produced as described in DE-A 25 40 279 or in a similar manner. In order to load the catalyst support with a chromium compound, the xerogel obtained is loaded with the desired amount of chromium by means of a 0.05 to 15% by weight solution of a chromium compound which is essentially converted into a chromium(VI) compound under the reaction conditions mentioned below, in an alcohol comprising a maximum of 20% by weight of water, by evaporation of the alcohol, and the resultant product is heated at temperatures in the range from 300 to 1100xc2x0 C. for a period of from 10 to 1000 minutes in a water-free gas stream which comprises at least 10% by volume of oxygen.
The said supported chromium catalysts have a number of disadvantages. It has been found that the chromium compound attaches principally to the outer regions of the catalyst support, while the inner regions are wetted to a much lesser extent. However, since the inner regions of the catalyst particles also participate in the catalysis, an inhomogeneous chromium distribution of this type is highly undesirable. In addition, chromium agglomerates formed due to locally limited deposition of chromium compounds on the catalyst surface are frequently observed.
The object of the invention is to indicate a method for producing a supported chromium catalyst which has a more uniform chromium distribution over the catalyst support particle cross section and which substantially avoids the formation of chromium agglomerates.
The object is achieved in accordance with the invention by a method for producing supported chromium catalysts by loading a xerogel support with chromium by
a) adding to the xerogel support a volume of a 0.025 to 15% by weight solution of a chromium compound or a volume of a solution comprising from 0.025 to 7.8% by weight of Cr which is essentially converted into a chromium(VI) compound on heating under oxidizing conditions at temperatures in the range from 300 to 1100xc2x0 C. for a period of from 10 to 1000 minutes in a solvent which comprises a maximum of 20% by weight of water, and subsequently
b) evaporating the solvent.
The invention is then characterised in that the volume of the chromium salt solution employed is smaller than the pore volume of the xerogel support. Xerogels are gels which have lost their liquid in some way (through evaporation, squeezing or aspiration), with the spatial arrangement of the network being changed in such a way that the separations between the structural elements only have dimensions of atomic separations. Xerogel supports can be produced based on silica gel as described in DE-A 25 40 279. Besides xerogel supports based on silica gel, however, xerogel supports based on aluminium phosphate or xerogel supports based on aluminium oxide/silicon oxide cogels or aluminium silicate gels are also suitable (cf. U.S. Pat. No. 2,825,721 or U.S. Pat. No. 2,930,789). When selecting the xerogel supports, those having a large pore volume of up to 3 ml/g are preferred. Suitable solvents for the chromium compound are, for example, alcohols or C3-C5-alkanones. C1-C4-alcohols are preferred, in particular methanol.
The details of the impregnation are given, for example, in DE-A 25 40 279. Suitable chromium compounds are, for example, chromium trioxide, chromium hydroxide and soluble salts of trivalent chromium with an organic or inorganic acid, such as acetate, oxalate, sulphate or nitrate. Particular preference is given to salts of acids which, on activation, are essentially converted into chromium(VI) without leaving a residue, such as chromium(III) nitrate nonahydrate. A volume of a solvent which comprises the chromium compound is added to the xerogel support. The volume of this chromium solution is smaller than the pore volume of the xerogel support.
The pore volume of the xerogel support is determined by the method described in the working example (see Table 1). The volume of the solvent is preferably less than 95%, especially less than 90%, in particular from 75 to 85%, of the pore volume of the xerogel support.
The xerogel support and the chromium solution (if desired with addition of fluorinating agents, such as ASF, ammonium hexafluorosilicate) are subsequently mixed, for example in a double-cone mixer. The solvent and any water present therein are distilled off at temperatures in the range from 20 to 200xc2x0 C., preferably from 100 to 150xc2x0 C., and pressures in the range from 1.3 mbar to 1 bar, preferably from 300 to 900 mbar.
The catalyst is activated (if desired with addition of fluorinating agents, such as ASF, ammonium hexafluorosilicate) under oxidizing conditions, for example in a water-free gas stream which comprises at least 10% by volume of oxygen, preferably at temperatures in the range from 300 to 1100xc2x0 C., particularly preferably at temperatures of from 500 to 800xc2x0 C., over a period of preferably from 60 to 1000 minutes, particularly preferably from 200 to 800 minutes, for example in a fluidised bed through which a stream of air flows.
The method according to the invention results in a catalyst having a chromium content of from 0.1 to 3% by weight, preferably from 0.7 to 1.5% by weight, particularly preferably from 0.9 to 1.2% by weight.
This is suitable for the homopolymerisation and copolymerisation of unsaturated compounds. The unsaturated compounds employed are preferably ethene or C3- to C8-xcex1-monoolefins.
It was hitherto not known that the amount of solvent in which the chromium compound is dissolved influences the properties of the supported chromium catalyst. Hitherto, at least the amount of solvent which corresponds to the pore volume of the support was used. This is because the person skilled in the art expects that all pores must be filled for an optimum chromium distribution. Thus, for example in U.S. Pat. No. 5,231,066,bimodal silica gel particles were impregnated with a chromium solution whose volume was a multiple of the pore volume of the support. U.S. Pat. No. 4,209,603 likewise disclosed an improved supported chromium catalyst for the polymerisation of a-alkenes during the impregnation of which an excess of solvent was employed.
In the production of supported chromium catalysts for the production of lubricants, xerogel supports based on silica gel were impregnated with solutions of a chromium compound in acetic acid whose volume corresponded to the pore volume of the support (U.S. Pat. No. 4,967,029 and WO 95/18782).
Surprisingly, it has now been found that a further reduction in the volume of the impregnating agent improves the homogeneity of the chromium distribution. It has been demonstrated by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) that on use of the method according to the invention, a greater proportion of chromium is bound in the interior of the xerogel support particle. In this way, a more homogeneous distribution of the chromium is thus achieved. In addition, the reduction in the volume of the impregnating agent has the consequence, to the complete surprise of experts, that no wall coatings are formed in the impregnation reactor and in the drying apparatus.
The invention furthermore relates to a supported chromium catalyst which is obtainable by a method according to the invention. This catalyst can be used in accordance with the invention for the polymerisation of ethene and/or of C3-C10-xcex1-monoolefins. Owing to the improved production method, the catalyst according to the invention has increased productivity and results in polymers having improved mechanical properties, for example increased stress cracking resistance. The invention furthermore relates to a method for the polymerisation of ethene and/or of C3-C10-xcex1-monoolefins in which a supported chromium catalyst according to the invention is used as catalyst.
The invention furthermore relates to polyethylenes obtainable by the above-mentioned method.